When a radio frequency (RF)/intermediate frequency (IF) device or system is calibrated or analyzed, a signal of a known spectral content is provided to the device or system and the resulting signal is then analyzed to estimate the response of the device or system. Signals with precisely known amplitude and phase relationships may be readily created in the digital domain and converted into the analog domain through digital-to-analog converters (DACs). Such signals may be created utilizing a variety of methods and devices including arbitrary waveform generators that access waveform samples from memory in a circular manner.
Frequently, a signal consisting of a plurality of discrete sinusoids of known amplitude and phase are utilized. These signals typically are periodic with a period T. Fourier analysis indicates that the spectrum of such periodic signals will be composed of discrete tones spaced in the frequency domain at integer multiples of 1/T. Power spectrum 100 of such a signal is shown in FIG. 1 where ωp=2π/T. Spectrums such as power spectrum 100 are commonly referred to as “comb” spectrums, because the tones resemble the teeth of a comb.
As shown in FIG. 2, these signals may be used as stimuli in system 200 to perform RF/IF testing. System 200 includes stimulus generator 201 that generates the desired signal in the digital domain. The digital I and Q symbols are provided to DACs 202 to convert the signal into the analog domain. The analog channels of the periodic signal may be provided to IQ modulator 203 (and possibly up/down converters which are not shown) to be translated to a desired RF/IF frequency. IQ modulator 203 mixes the received analog channels with an oscillator signal of frequency ωLO from local oscillator (LO) 204. The modulated signal is provided to RF/IF channel 205. RF/IF channel 205 is the communication medium, system, or device being tested or calibrated. RF/IF channel 205 may also include any suitable up-converters and/or down-converters. As discussed herein, a “channel” shall refer to any system, device, communication medium, or combination thereof that may transmit and/or process a suitable signal in a manner appropriate for measurement, analysis, or calibration.
The signal that results after communication through and/or processing by RF/IF channel 205 may then be converted to the digital domain by analog-to-digital converter (ADC) 206. Alternatively, the resulting signal may be processed by an IQ demodulator (not shown) before conversion into the digital domain. The digital version of the resulting signal may then be analyzed by signal analysis system 207. In general, the frequency content of the resulting signal is determined by employing a Discrete Fourier Transform (e.g., the Fast Fourier Transform (FFT)). From the frequency representation of the resulting signal, the frequency response of RF/IF channel 205 at the corresponding frequencies (which may be translated) may be determined.
It shall be appreciated that if IQ modulator 203 does not maintain the desired amplitude and phase relationships of the stimulus signal, errors in the measured frequency response will occur. In practice, IQ modulators commonly suffer from several dominant impairments such as LO leakage and amplitude and phase imbalance. LO leakage manifests itself as an undesired spectral spur at the local oscillator frequency (ωLO). LO leakage may be modeled as undesired DC offsets in the I and Q inputs. Amplitude imbalance occurs when the gain of the I channel and the gain of the Q channel are not equal. Phase imbalance occurs when the quadrature split of the LO signal for multiplication by the I and Q signals is not precisely at 90 degrees. As known in the art, amplitude and phase imbalance are manifested through the generation of a spectrally inverted image of the desired signal.
An example of artifacts generated by IQ modulator impairments is shown in FIG. 3. Impaired spectrum 300 comprises three spectral components 301, 302, and 303 that result from the application of the complex excitation associated with an Upper Sideband (USB) tone at frequency ωS to an impaired IQ modulator that utilizes a local oscillator of frequency ωLO. Spectral component 302 (at ωS+ωLO) is the desired spectral component to be produced by the IQ modulation. However, impaired spectrum 300 further comprises spectral component 301 (located at ωLO) due to the LO leakage of the IQ modulator. Spectral component 303 is observed at the image frequency (ωS−ωLO) as the result of the amplitude and/or phase imbalance of the IQ modulator.
The performance of IQ modulators is commonly characterized by the level of suppression of the undesired image as a measure of the amplitude and phase balance and the amount of the LO feedthrough. Greater suppression of these artifacts is associated with better modulators. Common values for the suppression of these terms is 20-30 dB suppression of the LO signal and 20-40 dB suppression for the inverted image. However, for some measurements or calibration functions, these levels need to be reduced to 40-50 dB (or greater) to achieve the desired accuracy. The cost (if even possible) of implementing modulators satisfying these levels of suppression may be prohibitive.